The invention relates to contacting shaley subterranean earth formations with drilling or treating fluids which might cause borehole instability problems. More particularly, the invention relates to identifying and quantifying the chemical swelling effect of a fluid on a shale-containing subterranean earth formation.
A recently filed patent (U.S. Pat. No. 4,142,595, filed Feb. 13, 1978) on a drilling fluid containing flax seed gum and potassium or ammonium salts, says, "Drilling of shale is a very old problem. One view of U.S. patent literature indicates that about 800 different substances have been disclosed for drilling shales in the patents issuing prior to 1954" (column 1, lines 14 to 17). The Principles of Drilling Mud Control, 12th edition, edited by a subcommittee of the API Southern District Study Committee on drilling fluids and published by Petroleum Extension Service, 1969, discusses the problems of drilling into shales (on pages 43 to 48) and indicates that . . . "Maintaining hole stability when drilling shale sections can be troublesome. No simple solution exists, but good drilling practices combined with good mud practices are helpful." In discussing good mud practices, it is stated that . . . "An ideal fluid would be one that does not alter the shales adversely, for once a problem has been created it is difficult to stop . . . "
U.S. Pat. No. 3,646,997 describes a procedure for determining compatibility of a well fluid with a subterranean shale by determining the direction and extent of water migration (due to adsorption or desorption within the shale) between the well fluid and the shale. With respect to such a procedure, the patent indicates that . . . "Laboratory tests performed at reservoir conditions of temperature and pressure when compared with adsorption tests conducted under atmospheric conditions, indicate that atmospheric tests are sufficiently accurate for most practical purposes." (column 5, lines 64-68).